Hypercholesterolemia is known to be one of the prime risk factors for ischemic cardiovascular disease, such as arteriosclerosis. Bile acid sequestrants have been used to treat this condition; they seem to be moderately effective but they must be consumed in large quantities, i.e. several grams at a time and they are not very palatable.
MEVACOR.RTM. (lovastatin) and ZOCOR.RTM. (simvastatin), now commercially available, are members of a group of very active antihypercholesterolemic agents that function by limiting cholesterol biosynthesis by inhibiting the enzyme HMG-CoA reductase.
Squalene synthase (also called squalene synthetase) is the enzyme involved in the first committed step of the de novo cholesterol biosynthetic pathway. This enzyme catalyzes the reductive dimerization of two molecules of farnesyl pyrophosphate to form squalene. The inhibition of this committed step to cholesterol should leave unhindered biosynthetic pathways to ubiquinone, dolichol and isopentenyl t-RNA.
Previous efforts at inhibiting squalene synthase have employed pyrophosphate or pyrophosphate analog containing compounds such as those described in P. Ortiz de Montellano et al., J. Med. Chem. 20, 243 (1977), E. J. Corey and R. Volante, J. Am. Chem. Soc., 98, 1291 (1976), and U.S. Pat. No. 5,025,003 to S. Billet. U.S. Pat. No. 4,871,721 to S. Billet describes isoprenoid(phosphinylmethyl) phosphonates as inhibitors of squalene synthase.
U.S. Pat. Nos. 5,096,923; 5,026,554; and 5,102,907 and U.S. patent applications Ser. Nos. 496,734 filed Mar. 21, 1990, and 698,766 filed May 10, 1991 disclose non-phosphorus-containing substituted 2,8-dioxabicyclo[3.2.1]octane derivatives useful as squalene synthase inhibitors. Furthermore, J. Antibiotics 45: 639-658 (1992) describe a squalene synthase inhibitor called squalestatin.
Recently it has been shown that certain natural product nonphosphorous containing inhibitors of squalene synthase and their esters are useful in inhibiting fungal growth. This utility is described in U.S. Pat No. 5,026,554.
The present invention is directed to the use of compounds of structural formula (I) which are squalene synthase inhibitors for the inhibition of fungal growth.
The present invention is also directed to the use of compounds of structural formula (I) which are inhibitors of farnesyl protein transferase for inhibition of farnesylation of the oncogene protein Ras and the treatment of cancer.
The Ras gene is found activated in many human cancers, including colorectal carcinoma, exocrine pancreatic carcinoma, and myeloid leukemias. Biological and biochemical studies of Ras action indicate that Ras functions like a G-regulatory protein, since Ras must be localized in the plasma membrane and must bind with GTP in order to transform cells (Gibbs, J. et al., Microbiol. Rev. 53:171-286 (1989)). Forms of Ras in cancer cells have mutations that distinguish the protein from Ras in normal cells.
At least 3 post-translational modifications are involved with Ras membrane localization, and all 3 modifications occur at the C-terminus of Ras. The Ras C-terminus contains a sequence motif termed a "CAAX" or "Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa" box (Aaa is an aliphatic amino acid, the Xaa is any amino acid) (Willumsen et al., Nature 310:583-586 (1984)). Other proteins having this motif include the Ras-related GTP-binding proteins such as Rho, fungal mating factors, the nuclear lamins, and the gamma subunit of transducin.
Farnesylation of Ras by the isoprenoid farnesyl pyrophosphate (FPP) occurs in vivo on Cys to form a thioether linkage (Hancock et al., Cell 57:1167 (1989); Casey et al., Proc. Natl. Acad, Sci. USA 86:8323 (1989)). In addition, Ha-Ras and N-Ras are palmitoylated via formation of a thioester on a Cys residue near a C-terminal Cys farnesyl acceptor (Gutierrez et al., EMBO J. 8:1093-1098 (1989); Hancock et al., Cell 57:1167-1177 (1989)). Ki-Ras lacks the palmitate acceptor Cys. The last 3 amino acids at the Ras C-terminal end are removed proteolytically, and methyl esterification occurs at the new C-terminus (Hancock et al., ibid). Fungal mating factor and mammalian nuclear lamins undergo identical modification steps (Anderegg et al., J. Biol. Chem. 263:18236 (1988); Farnsworth et al., J. Biol. Chem. 264:20422 (1989)).
Inhibition of Ras farnesylation in vivo has been demonstrated with lovastatin (Merck & Co., Rahway, N.J.) and compactin (Hancock et al., ibid; Casey et al., ibid; Schafer et al., Science 245:379 (1989)). These drugs inhibit HMG-CoA reductase, the rate limiting enzyme for the production of polyisoprenoids and the farnesyl pyrophosphate precursor. It has been shown that a farnesyl-protein transferase using farnesyl pyrophosphate as a precursor is responsible for Ras farnesylation. (Reiss et al., Cell, 62:81-88 (1990); Schaber et al., J. Biol. Chem., 265:14701-14704 (1990); Schafer et al., Science, 249:1133-1139 (1990); Manne et al., Proc. Natl. Acad. Sci. USA, 87:7541-7545 (1990)).
Inhibition of farnesyl-protein transferase and, thereby, of farnesylation of the Ras protein, blocks the ability of Ras to transform normal cells to cancer cells. Surprisingly, the compounds of the invention inhibit Ras farnesylation and, thereby, generate soluble Ras which, as indicated infra, can act as a dominant negative inhibitor of Ras function. While soluble Ras in cancer cells can become a dominant negative inhibitor, soluble Ras in normal cells would not be an inhibitor.
A cytosol-localized (no Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa box membrane domain present) and activated (impaired GTPase activity, staying bound to GTP) form of Ras acts as a dominant negative Ras inhibitor of membrane-bound Ras function (Gibbs et al., Proc. Natl. Acad. Sci. USA 86:6630-6634 (1989)). Cytosol-localized forms of Ras with normal GTPase activity do not act as inhibitors. Gibbs et al., ibid, showed this effect in Xenopus octyes and in mammalian cells.
Administration of compounds of the invention to block Ras farnesylation not only decreases the amount of Ras in the membrane but also generates a cytosolic pool of Ras. In tumor cells having activated Ras, the cytosolic pool acts as another antagonist of membrane-bound Ras function. In normal cells having normal Ras, the cytosolic pool of Ras does not act as an antagonist. In the absence of complete inhibition of farnesylation, other farnesylated proteins are able to continue with their functions.
Farnesyl-protein transferase activity may be reduced or completely inhibited by adjusting the compound dose. Reduction of farnesyl-protein transferase enzyme activity by adjusting the compound dose would be useful for avoiding possible undesirable side effects such as interference with other metabolic processes which utilize the enzyme.
These compounds are inhibitors of farnesyl-protein transferase. Farnesyl-protein transferase utilizes farnesyl pyrophosphate to covalently modify the Cys thiol group of the Ras CAAX box with a farnesyl group. Inhibition of farnesyl pyrophosphate biosynthesis by inhibiting HMG-CoA reductase blocks Ras membrane localization in vivo and inhibits Ras function. Inhibition of farnesyl-protein transferase is more specific and is attended by fewer side effects then is the case for a general inhibitor of isoprene biosynthesis.
Previously, it has been demonstrated that tetrapeptides with the CAAX sequence inhibit Ras farnesylation (Schaber et al., ibid: Reiss et. al., ibid; Reiss et al., PNAS, 88:732-736 (1991)). However, the reported inhibitors of farnesyl-transferase are metabolically unstable or inactive in cells.
Pharmaceutical compositions containing the compounds of this invention and methods of treatment utilizing these compositions for use in inhibiting farnesyl-protein transferase and farnesylation of the oncogene protein Ras are described herein.